Among components of a radio communication apparatus, a power amplifier for transmission used in the radio communication apparatus, in particular, consumes power most. Thus, reduction of power consumption by the power amplifier (Power Amplifier, PA) is regarded as the most important challenge for development of the radio communication apparatus.
In a recent communication standard, a linear modulation method has become a mainstream for spectrum efficiency improvement. In the linear modulation method, signal amplitude indicates a temporal fluctuation. Being different from a frequency modulation method using a constant signal amplitude, the linear modulation method imposes a rigorous requirement against distortion of a transmission signal. As a measure against this distortion, average output power of the transmission signal is set such that instantaneous maximum output (peak) power of the transmission signal is not more than the saturation output of a power amplifier. The distortion of the transmission signal can be thereby reduced.
However, generally, power efficiency of the power amplifier is reduced as the average output power is reduced to be low with respect to the saturation output power (as a back-off amount is increased). The power efficiency of the power amplifier is determined by a ratio between transmission power output from the power amplifier and power (power consumption) supplied to the power amplifier from a power source. Reduction of the power efficiency unnecessarily increases the power consumption with respect to the transmission power of a desired level. Thus, a method of reducing power consumption of the power amplifier even if a back-off amount is set has been developed.
An example of a method of maintaining power efficiency of a power amplifier to be high and reducing power consumption of the power amplifier regardless of a temporal amplitude variation of a linearly modulated signal is disclosed in Patent Literature 1. FIG. 30 is a block configuration diagram illustrating a block configuration of a transmission apparatus described in Patent Literature 1. In the transmission apparatus in FIG. 30, a linearly modulated signal supplied to a terminal 11 is converted to two frequency-modulated signals V1 and V2 through a series-parallel converter 12, filters 13 and 14, and an operation circuit 15. The frequency-modulated signals V1 and V2 are respectively supplied to power amplifiers 16 and 17. Since amplitudes of the frequency-modulated signals V1 and V2 each have a constant value that does not temporarily vary, there is no need for setting back-off for each of the power amplifiers 16 and 17. High power efficiency is thereby maintained. Frequency-modulated signals S1 and S2 amplified and output by the power amplifiers 16 and 17 are supplied to a power combiner 18 through terminals 2 and 3. The power combiner 18 combines the signals S1 and S2 to regenerate an amplified signal of the linearly modulated signal supplied to the terminal 11 and outputs the regenerated signal to a terminal 4 and a load 9.
A differential signal between the frequency-modulated signals S1 and S2 is output to a terminal 5 from the power combiner 18. Usually, the terminal 5 is terminated by a resistor, so that power of the differential signal becomes a loss due to the termination by the resistor. Then, in the method illustrated in FIG. 30, a rectifier circuit 20 and filters 19 and 21 are installed after the terminal 5. The differential signal output to the terminal 5 is converted into direct-current power to be output to a terminal 22. The direct-current power output to the terminal 22 is reused as power to be supplied to the power amplifiers 16 and 17, thereby reducing wasteful power consumption. Power saving of the PAs is implemented by the above-mentioned method.
Another example of the method of maintaining power efficiency of a power amplifier to be high and reducing power consumption of the power amplifier regardless of a temporal amplitude variation of a linearly modulated signal is disclosed in Patent Literature 2. FIG. 31 is a block configuration diagram showing a block configuration of a transmission apparatus described in Patent Literature 2.
In the transmission apparatus in FIG. 31, a phase-modulated signal obtained by excluding an amplitude component from a transmission signal is generated by a signal generation unit 31 and an angle modulation unit 32. The phase-modulated signal is supplied to each of power amplifiers 551 and 552 through a directional coupler 61. Since the phase-modulated signal has a constant amplitude, there is no need for setting back-off for each of the power amplifiers 551 and 552. High power efficiency is thereby maintained. Further, an amplitude signal obtained by removing a phase component from the transmission signal is amplified by a variable gain amplifier 28 and a regulator 34. Then, the amplitude signal is supplied to power supply terminals of the power amplifiers 551 and 552. The phase-modulated signal is supplied to each of the power amplifiers 551 and 552, and the power supply terminals are modulated by the amplitude signal. Thus, the transmission signal including the amplitude component and the phase component is regenerated to be output to an output terminal 37 through a directional coupler 62. When power of the transmission signal is large, a switch 53 is turned on to supply the power to the power amplifier 551, and the power amplifiers 551 and 552 are both operated. When the power of the transmission signal is small, the switch 53 is turned off, thereby stopping supply of the power to the power amplifier 551, and the power amplifier 552 is operated. Power saving is thereby attempted.
In the transmission apparatus in FIG. 31, a differential signal between outputs of the power amplifiers 551 and 552 is output to a power reuse unit 63 from the directional coupler 62. Usually, power of the differential signal between the outputs of the power amplifiers 551 and 552 is consumed wastefully. Then, in the method in FIG. 31, the differential signal is converted into direct-current power by the power reuse unit 63 to be output to a terminal 33. The direct-current power supplied to the terminal 33 is reused as the power to be supplied to the power amplifier 551 and power to be supplied to the power amplifier 552 through the regulator 34. Wasteful power consumption is thereby reduced. Power saving of the PAs is implemented by the above-mentioned method.
Still another example of the method of maintaining power efficiency of a power amplifier to be high and reducing power consumption of the power amplifier regardless of a temporal amplitude variation of a linearly modulated signal is disclosed in Patent Literature 3. FIG. 32 is a block configuration diagram showing a block configuration of a transmission apparatus described in Patent Literature 3.
In the transmission apparatus in FIG. 32, a baseband is output from a data generator 101. The baseband signal is converted to a digital signal through a vector data converter 102 and a modulator 103 to be supplied to a power amplifier 104. Since the digital signal has a constant amplitude, there is no need for setting back-off for the power amplifier 104. High power efficiency is thereby maintained. The digital signal is amplified by the power amplifier 104 and is then output. Then, the digital signal is supplied to a filer 108 through an isolator 105. The filter 108 converts the digital signal to an RF signal obtained by removing a quantization error of the digital signal and superimposing the baseband signal thereon, and outputs the RF signal to a terminal 109.
The quantization error removed by the filer 108 is output to a terminal c of the isolator 105. Usually, the terminal c of the isolator 105 is terminated by a resistor, and power of the quantization error becomes a loss due to the termination by the resistor. Then, in the method in FIG. 32, a power reuse unit 106 is connected to the terminal c of the isolator 105, and the power of the quantization error is converted to direct-current power and is reused as a portion of direct-current power to be supplied from a power supply unit 107 to the power amplifier 104. Power loss reduction is thereby provided.
An example of a method of providing power saving of a transmission apparatus by reuse of loss power is disclosed in Patent Literature 4. FIG. 33 is a block configuration diagram showing a block configuration of the transmission apparatus described in Patent Literature 4.
In the transmission apparatus in FIG. 33, an RF signal output from a signal source 270 is amplified by a power amplifier 220. The RF signal is then output to an antenna 210 through an isolator 240. A portion of the RF signal supplied to the antenna 210 is reflected due to a matching deviation. Unless the isolator 240 is installed, reflected power of the RF signal becomes a loss at an output terminal of the power amplifier 220. Then, in the transmission apparatus in FIG. 33, the isolator 240 is installed, the reflected power of the RF signal is output to a recovery unit 230 through the isolator 240, so that the reflected power of the RF signal is converted to direct-current power by the recovery unit 230. A switching unit 260 selects the recovery unit 230 or a battery unit 250, and supplies the direct-current power output from the recovery unit 230 or direct-current power output from a battery unit 250 to the power amplifier 220, a signal source 270, and a configuration unit 280. The configuration unit 280 is a circuit in general necessary for configuring a radio apparatus. As mentioned above, the transmission apparatus in FIG. 33 reuses the power reflected from the antenna that may usually become the loss, as power to be supplied to the circuit. Power saving of the transmission apparatus is thereby implemented.
Another example of implementing power saving of a transmission apparatus by reuse of loss power is disclosed in Patent Literature 5. FIG. 34 is a block configuration diagram showing a block configuration of the transmission apparatus described in Patent Literature 5.
In the transmission apparatus in FIG. 34, a communication processing unit 324 operates upon receipt of supply of power from a power supply unit 320, and transmits a data signal received from a control unit 316. The communication processing unit 324 outputs an RF signal for transmission to a transmitting antenna 328. A portion of the RF signal emitted into the air from the transmitting antenna 328 is supplied to a power recovery antenna 304. The RF signal supplied to the power recovery antenna 304 is converted into direct-current power by a rectifier circuit 308, and the direct-current power is supplied to a power use unit 312. The power use unit 312 is a circuit in general configured to use power. As mentioned above, in the transmission apparatus in FIG. 34, a portion of the power of the RF signal emitted from the transmitting antenna 328 into the air is reused, thereby attempting power saving of the transmission apparatus.
In recent radio technologies, while power saving of a transmission apparatus is a major challenge, implementation of radio communication at a higher speed is also demanded. As shown in Non Patent Literature 1, a Carrier aggregation technology (hereinafter referred to as a CA technology) configured to aggregate and use a plurality of fragmented bands is employed for implementing such high-speed radio communication. In this CA technology, the plurality of bands are aggregated to secure a wide band. A transmission speed can be thereby increased.
In an Inter-band Non-contiguous CA mode in which respective carrier frequencies are greatly spaced apart to each other (a difference Δf between the respective carrier frequencies is sufficiently larger than a modulated bandwidth fBB of each carrier RF signal), by performing simultaneous communication using a plurality of the carrier frequencies having different propagation characteristics, stability of the communication can be improved. Further, by applying the CA technology, communication that accommodates a case where band allocation for a plurality of operators is non-contiguous, or a case where the plurality of operators share a band can be performed.
In a communication system using the CA technology, a transmission apparatus and a transmission method configured to transmit RF signals of a plurality of bands (bands) are needed. Improvement in power efficiency is demanded for such a transmission apparatus as well.
FIG. 35 is a functional configuration diagram of a transmitter according to the technique described in Patent Literature 6. The transmitter illustrated in FIG. 35 has a function of transmitting RF signals of a plurality of bands and also has a function of improving power efficiency by application of a polar modulation technique.
Specifically, in the transmitter illustrated in FIG. 35, a modulated signal generated by a modulated signal generator 461 is converted to a signal in a polar coordinate system from a signal in an orthogonal coordinate system to be separated into a PM signal including phase information and an AM signal including amplitude information. The PM signal obtained by the separation is used for phase modulation for a frequency generator 411 through a PM control unit 463. Similarly, the AM signal is used for power supply modulation for PAs 421 and 431 through a power supply modulator 464. That is, the polar modulation technique is applied which increases or decreases power to be supplied from the power supply modulator 464 to each of the PAs 421 and 431, as well, according to an increase or a decrease in power output from each of the Pas 421 and 431. Reduction in the power efficiency is suppressed even in a high back-off state where average output power is set to be low.
The transmitter illustrated in FIG. 35 includes path selection switches 414 and 441 configured to perform switching between a GSM (registered trademark) (Global System for Mobile Communications) signal path 420 in which the PA 421 is provided and a UMTS signal path 430 in which the PA 431 is provided. The PA 421 amplifies an RF signal in a (GSM) communication system for a carrier frequency fc1, while the PA 431 amplifies an RF signal in a (Universal Mobile Telecommunications System, UMTS) communication system for a carrier frequency fc2. When communication is performed in the communication system for the carrier frequency fc1, switching of the path selection switches 414 and 441 is performed by a control signal from a controller 415 such that the RF signal is supplied to and output from the PA 421. When communication is performed in the communication system for the carrier frequency fc2, switching of the path selection switches 414 and 441 is performed by the control signal from the controller 415 such that the RF signal is supplied to or output from the PA 431.
The transmitter illustrated in FIG. 35 does not accommodate the CA technology whereby two desired frequency components fc1 and fc2 are simultaneously output. However, the transmitter illustrated in FIG. 35 has a multi-band operation function configured to temporally perform switching between the frequency components fc1 and fc2 to operate one of the frequencies.
A technique where the polar modulation technique is applied to each PA to maintain power efficiency to be high even if average output power has been set to be low is also disclosed in each of Patent Literature 7, Patent Literature 8, Patent Literature 9, and Patent Literature 10. In this polar modulation technique, as in the transmitter illustrated in FIG. 35, the PAs, being equal in number to bands for use, are provided. Then, the PAs are respectively assigned for each band. A band selection switch is installed for an input or an output of each PA. The switch is switched such that an RF signal is supplied to and output from the PA corresponding to the band during use, thereby controlling power to be supplied from a power source.    PTL 1    JP Patent Kokai Publication No. JP2000-349575A    PTL 2    JP Patent Kohyo Publiciation No. JP2008-539601A    PTL 3    JP Patent Kokai Publication No. JP2005-287017A    PTL 4    JP Patent No. 4413244    PTL 5    JP Patent Kokai Publication No. JP2011-205793A    PTL 6    JP Patent Kokai Publication No. JP2006-324878A    PTL 7    JP Patent Kohyo Publication No. JP2011-512098A    PTL 8    JP Patent Kokai Publication No. JP2005-244826A    PTL 9    JP Patent Kokai Publication No. JP2006-270923A    PTL 10    JP Patent Kokai Publication No. JP2008-205821A    NON PATENT LITERATURE    NPL 1    Nobuhiko Miki, et. al., “Carrier Aggregation for Bandwidth Extension in LTE-Advanced,” NTT DoCoMo technical Journal, Vol. 18, No. 2    NPL 2    S. Kousai, et. Al., “An Octave-Range Watt-Level, Fully-Integrated CMOS Switching Power Mixer Array for Linearization and Back-Off-Efficiency,” IEEE Journal of Solid-State Circuits, vol. 44, no. 12, pp. 3376-3392, 2009    NPL 3    P. Saad, et. al., “Design of a Highly Efficient 2-4 GHz Octave Bandwidth GaN-HEMT Power Amplifier,” IEEE Transactions on Microwave Theory and Techniques, vol. 58, no. 7, pp. 1677-1685, 2010